Agronomic consequences of tractor wheel compaction on a clay soil

In southern New South Wales, Australia, farming operations using tractors often occur when the soils are moist and prone to soil compaction. However, the extent of soil compaction and its relative impact on crop yield have not been quantified in the region. In this experiment, re-compaction due to tractor wheel traffic in a sodie brown clay (Vertisol) was monitored under simulated controlled traffic conditions after removal of a pre-existing subsoil pan by deep tillage. Soil physical properties under wheel tracks were compared to those between wheel tracks in terms of bulk density, penetrometer resistance, water content, air-filled porosities and changes in "least limiting water range". Differences in the growth and yield of canola (Brassica napus) and wheat (Triticum aestivum) in the two areas were also measured. Although deep ripping increased canola yield by 20% (from 2.0 to 2.4 t ha(-1)), reformation of a compaction pan under the wheel tracks was already detected in the first season of cropping. In the second cropping year, soil in the 0.05-0.10 m layer under wheel tracks had significantly higher penetrometer resistance (> 2000 kPa) and bulk density (1.5-1.58 Mg m(-3)) and lower air-filled porosity (0.07-0.09 m(3) m(-3)) compared to that measured between wheel tracks (< 1000 kPa and 1.25-1.29 Mg m(-3), and 0.187-0.226 m(3) m(-3), respectively). The 'least limiting water ranged was essentially reduced to zero under wheel tracks and hence was unfavourable to plant roots. By contrast, favourable conditions were maintained in the area between wheel tracks throughout the whole available water range. This finding was supported by a significant reduction in canola and wheat root growth in the layer under the wheel tracks. While there was no difference in wheat yield (5.3-5.5 t ha(-1)), canola grain yield on the wheel track was only 34% of that between wheel tracks (1.1 t ha(-1) versus 3.2 t ha(-1)). The canola results highlight the potential loss in grain yield due to compaction by tractor wheel traffic and indicate the likely benefits of adopting controlled traffic in farming systems for the sodic brown clay soils of this region. However, to fully realise the benefits of controlled traffic on these soil types it may first be necessary to remove the underlying compaction generated by previous farming practices. Conversely, adoption of controlled traffic systems should greatly assist in slowing the rate of recompaction of cropping soils following deep ripping. (c) 2005 Elsevier B.V. All rights reserved.